Why aren’t my hero images appearing as thumbnails in the SERP?

I display hero images across the top of my blog pages. However, I use CSS images rendering through div tags and I read that google doesn’t typically index those.

So the images aren’t included as thumbnails in SERPs, see below.

Can you please confirm if there is a way to direct google to index the images like with an image sitemap?

Does an image have to be included in the IMG tag to be considered for a thumbnail in the SERP?

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